1. Field of Invention
The present invention relates to a low-pressure discharge lamp in which external electrodes are employed to drive an electrical gas discharge confined within a laminated envelope. More particularly the present invention relates to such a discharge lamp which could be utilized for the purpose of automotive rear lighting applications.
2. Description of Related Art
In the neon signage industry, the standard type of electrode employed in low-pressure discharge lamps is the internal electrode. Internal electrodes, as the name provides, are located within the glass tubing and typically consist of a metal shell coated with an emissive coating. A connection to an external power source is made via a wire which is glass-to-metal sealed in the tubing see generally W. Strattman, Neon Techniques, Handbook of Neon Sign and Cold Cathode Lighting, ST Publications, Inc., Cincinnati, Ohio (1997).
A significant problem associated with low-pressure discharge lamps comprising internal electrodes is a reduction in lifetime due to electrode failure resulting from bombardment of the electrode by gas ions, and sputtering away of material from the electrode. Further, failure in these discharge lamps is also associated with leakage at the glass-to-metal seal i.e., at the seal between the glass envelope and the electrode. This mode of failure is particularly true in discharge lamps having borosilicate-to-tungsten wire seals.
In contrast to internal electrodes, the activation of an ionizable gas by external electrodes eliminates the aforementioned destruction of electrodes, resulting in longer lamp life, i.e., external electrodes are on the outside of the glass tubing and therefore are not subject to bombardment by gas ions. The term “external electrodes” is meant to refer to electrodes that are not internal to a glass article containing an ionizable gas.
An additional feature of driving a discharge through external electrodes is that multiple separate channels can be driven in parallel, unlike driving a discharge through internal electrodes, which will only follow the path of least resistance.
Capacitive coupling to a low-pressure discharge, i.e., driving a discharge through external electrodes has been disclosed in U.S. Pat. No. 4,266,166 Proud et al.) and U.S. Pat. No. 4,266,167 (Proud et al.). U.S. Pat. No. 4,266,166 discloses a fluorescent lamp comprising a pear-shaped glass envelope with a reentrant cavity in the lamp envelope. An outer and inner conductor, typically a conductive mesh, is disposed on the outer surface of the envelope and on the reentrant cavity surface, respectively. Similarly, U.S. Pat. No. 4,266,167 discloses a fluorescent lamp comprising a pear-shaped glass envelope with a reentrant cavity. An outer conductor, typically a conductive mesh, is disposed on the outer surface of the lamp envelope, and an inner conductor, typically a solid conductive device, fills the reentrant cavity. Both patents disclose the use of a high frequency of operation, in the range of 10 MHz to 10 GHz.
A fluorescent lamp wherein a twin-tube lamp envelope comprises electrodes at or near the ends thereof for capacitive coupling to a low pressure discharge lamp is disclosed in U.S. Pat. No. 5,289,085 (Godyak et al.). Externally located electrodes comprising metal layers or bands at or near the ends of the tube envelope are disclosed. Frequencies in the range of 3 MHz to 300 MHz are suggested.
U.S. Pat. No. 5,041,762 (Hartai) discloses a luminous panel comprising a flat glass envelope formed from two plates of glass, the flat glass envelope comprising a gas discharge channel formed by machining a groove on the surface of the plates. Although the preferred embodiment discloses internal electrodes, electrodes of the capacitive type are also suggested.